The lining of blood vessels (endothelium) releases a variety of substances that cause the smooth muscle to relax. One important dilator is known as nitric oxide. To study whether nitric oxide is important in reactive hyperemia, we studied the effect of blocking nitric oxide production with L-NMMA in patients with normal coronary arteries and those with hardening of their coronary arteries (atherosclerosis). Reactive hyperemia can be caused by occluding blood supply to a limb temporarily which allows ischemia to occur and then the vasodilatation that follows ischemia is termed reactive hyperemia. The study demonstrated that nitric oxide released during dilation contributes significantly to the magnitude of dilation, and the inhibition of nitric oxide production significantly diminishes the degree of vasodilation that occurs during reactive hyperemia. This suggests that blood vessel dilation during stress is at least partly dependent on the release of nitric oxide.